1. Field of the Invention
The present invention relates to a nickel positive electrode for alkaline rechargeable batteries, and alkaline rechargeable batteries using the same positive electrode, more particularly nickel-metal hydride (Ni--MH) batteries.
2. Description of the Prior Art
Alkaline rechargeable batteries are batteries using alkaline electrolyte mainly composed of potassium hydroxide (KOH), and in almost all battery systems, the nickel positive electrode excellent in charge and discharge cycle performance is used. In the negative electrode, iron (Fe), zinc (Zn), cadmium (Cd) and others are used as active material, but because of the long charge and discharge cycle life and high reliability of sealed batteries, the main stream of alkaline batteries was nickel-cadmium (Ni--Cd) batteries using Cd negative electrode. Recently, using a hydrogen absorbing alloy capable of repeating absorption and desorption of hydrogen by charge and discharge, higher in capacity than in Cd negative electrode, in the negative electrode, the Ni--MH batteries are developed, and are expanding demands. In particular, they are expected to be applied widely, ranging from the small-sized power supply for portable appliances such as cellular telephone and camcorder, to large- and medium-sized power supply for electric vehicle (EV) and hybrid electric vehicle (HEV).
In these alkaline rechargeable batteries, in order to reduce further in size and weight, it is always demanded to enhance the energy density. Means for increasing the capacity to enhance the energy density in a specific battery system may be roughly classified into two types. First, in limited battery dimensions, the inner volume of the battery container is expanded as much as possible, while the volume of the materials and parts not directly responsible for charge and discharge reaction is decreased, and the active materials of the positive and negative electrodes contributing to reaction are used in good balance and as much as possible. For example, in the cylindrical cell, using the so-called DI can having a thick bottom and a thin side wall, the inner volume of the cell is expanded, and a thin separator is used, so that the occupied volume of positive and negative electrodes is increased. Hitherto, owing to the excellent high rate discharge characteristics and long life, using a thin porous sheet by sintering carbonyl nickel (Ni) powder as plaque, sintered electrodes having active materials impregnated therein were widely used, and by contrast, it has been attempted to increase the filling amount of active materials and enhance the capacity by employing foamed type electrodes filled with paste type active materials, using a foamed Ni sheet extremely high in porosity and large in pore volume as plaque, coated type electrodes coated with paste type active materials on one or both sides of a plaque which is a perforated Ni sheet or Ni plated steel sheet such as punching metal and expanded metal, and other paste type electrodes.
Other means for enhancing the capacity is to increase the of utilization of active materials of positive and negative electrodes contained in the cell. Herein, the utilization of active materials is expressed by percentage of the practical capacity to theoretical capacity of the contained active materials.
Cobalt (Co) is noted as one of effective additives for enhancing the utilization of nickel hydroxide, Ni(OH).sub.2, an active material of paste type nickel positive electrode of foamed type or coated type. Various methods of its addition have been proposed. Typical examples include the following: (1) a method of adding cobalt monoxide (CoO) (Japanese Laid-Open Patent Application No. Sho. 61-138458), (2) a method of adding at least one of CoO with particle size of 1/2 or less of the particle of Ni(OH).sub.2 of active material, beta- and alpha-cobalt hydroxide, and Co(OH).sub.2 (Japanese Laid-Open Patent Application No. Sho. 62-256366), and (3) a method of adding Co(OH).sub.2 with specific surface area of 20 m.sup.2 /g or less (Japanese Laid-Open Patent Application No. Sho. 62-66570), among others.
In the method of (1), CoO is easily oxidized in air, and is unstable and hard to handle. Yet, the addition effect of CoO is not exhibited unless it is let stand for a long time until the initial charge after composing the cell by injecting by using the nickel positive electrode to which CoO is added, and there was a problem in productivity.
In the method of (2), it is necessary to control the crystal structure of Co(OH).sub.2 of the additive, and there was a problem in supply of materials.
In the method of (3), when the specific surface area of Co(OH).sub.2 added for effectively enhancing the utilization of active materials of nickel positive electrode is increased nearly to 20 m.sup.2 /g, it becomes unstable in air. To the contrary, by using Co(OH).sub.2 of smaller specific surface area, the addition amount must be increased in order to enhance the utilization of positive electrode active material, which causes to increase the manufacturing cost.